Organic light-emitting diodes (OLEDs) are a promising technology for flat-panel displays and area illumination lamps. The technology relies upon thin-film layers of organic materials and electrodes coated upon a substrate and encapsulated with a cover. These layers of materials have differing refractive indices. In particular, the cover and substrate typically have a lower refractive index than the thin-film layers of materials that, in turn, have a lower refractive index than transparent electrodes. As is well known, because these layers have different refractive indices, much of the light output from the light-emissive layer in the OLED is contained within the device. Because the light emission from the OLED is emitted in all directions, some of the light is emitted directly from the device, some is emitted into the device and is either reflected back out or is absorbed, and some of the light is emitted laterally and trapped and absorbed by the various layers comprising the device. In general, up to 80% of the light may be lost in this manner.
Organic light emitting devices (OLED) generally can have two formats known as small molecule devices such as disclosed in U.S. Pat. No. 4,476,292 and polymer OLED devices such as disclosed in U.S. Pat. No. 5,247,190. Either type of OLED device may include, in sequence, an anode, an organic EL element, and a cathode. The organic EL element disposed between the anode and the cathode commonly includes an organic hole-transporting layer (HTL), an emissive layer (EL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the EL layer. Tang et al. (Appl. Phys. Lett., 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and U.S. Pat. No. 4,769,292) demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved.
Light is generated in an OLED device when electrons and holes that are injected from the cathode and anode, respectively, flow through the electron transport layer and the hole transport layer and recombine in the emissive layer. Many factors determine the efficiency of this light generating process. For example, the selection of anode and cathode materials can determine how efficiently the electrons and holes are injected into the device; the selection of ETL and HTL can determine how efficiently the electrons and holes are transported in the device, and the selection of EL can determine how efficiently the electrons and holes be recombined and result in the emission of light, etc. It has been found, however, that one of the key factors that limits the efficiency of OLED devices is the inefficiency in extracting the photons generated by the electron-hole recombination out of the OLED devices. Due to the high optical indices of the organic materials used, most of the photons generated by the recombination process are actually trapped in the devices due to total internal reflection. These trapped photons never leave the OLED devices and make no contribution to the light output from these devices.
A typical OLED device uses a glass substrate, a transparent conducting anode such as indium-tin-oxide (ITO), a stack of organic layers, and a reflective cathode layer. Light generated from the device is emitted through the glass substrate. This is commonly referred to as a bottom-emitting device. Alternatively, a device can include a substrate, a reflective anode, a stack of organic layers, and a top transparent electrode layer. Light generated from the device is emitted through the top transparent electrode. This is commonly referred to as the top-emitting device. In these typical devices, the index of the ITO layer, the organic layers, and the glass is about 2.0, 1.7, and 1.5 respectively. It has been estimated that nearly 60% of the generated light is trapped by internal reflection in the ITO/organic EL element, 20% is trapped in the glass substrate, and only about 20% of the generated light can actually emit from the device and perform useful functions.
Referring to FIG. 12, a prior-art bottom-emitting OLED has a transparent substrate 10, a first (transparent for bottom-emitting) electrode 12, one or more layers 14 of organic material, one of which is light-emitting, a second (reflective for bottom-emitting) electrode 16, a gap 18 and an encapsulating cover 20. The encapsulating cover 20 may be opaque and may be coated directly over the reflective electrode 16 so that no gap 18 exists. When a gap 18 does exist, it may be filled with polymer or desiccants to add rigidity and reduce water vapor permeation into the device. Light emitted from one of the organic material layers 14 can be emitted directly out of the device, through the substrate 10, as illustrated with light ray 1. Light may also be emitted and internally guided in the substrate 10 and organic layers 14, as illustrated with light ray 2. Alternatively, light may be emitted and internally guided in the layers 14 of organic material, as illustrated with light ray 3. Light rays 4 emitted toward the electrode 16 are reflected toward the substrate 10 and then follow one of the light ray paths 1, 2, or 3.
Referring to FIG. 13, a top-emitting OLED device as proposed in the prior art is illustrated having a substrate 10 (either reflective, transparent, or opaque), a first (reflective for top-emitting) electrode 12, one or more layers 14 of organic material, one of which is light-emitting, a second (transparent for top-emitting) electrode 16, a gap 18 and an encapsulating cover 20. The encapsulating cover 20 is transparent and may be coated directly over the electrode 16 so that no gap 18 exists. It has been proposed to fill the gap with polymeric or desiccating material. Such polymers and desiccants typically will have indices of refraction greater than or equal to that of the substrate 10 or encapsulating cover 20, and it is generally proposed to employ materials having indices of refraction matched to that of the encapsulating cover to reduce interlayer reflections. Light emitted from one of the organic material layers 14 can be emitted directly out of the device, through the encapsulating cover 20, as illustrated with light ray 1. Light may also be emitted and internally guided in the encapsulating cover 20 and organic layers 14, as illustrated with light ray 2. Additionally, light may be emitted and internally guided in the layers 14 of organic material, as illustrated with light ray 3. Light rays 4 emitted toward the electrode 12 are reflected toward the cover 20 and follow one of the light ray paths 1, 2, or 3. In some prior-art top-emitting embodiments, the electrode 12 may be opaque and/or light absorbing. The Figures are not drawn to scale because the organic layers 14 and electrodes 12 and 16 are so thin (on the order of 100 nm) with respect to substrate 10 and cover 20 (typically thicknesses of around 1 mm). Hence, the distance that light travels through the organic layers 14 and electrodes 12 and 16 is greatly exaggerated in the Figures.
A variety of techniques have been proposed to improve the out-coupling of light from thin-film light emitting devices. For example, diffraction gratings have been proposed to control the attributes of light emission from thin polymer films by inducing Bragg scattering of light that is guided laterally through the emissive layers; see “Modification of polymer light emission by lateral microstructure” by Safonov et al., Synthetic Metals 116, 2001, pp. 145-148, and “Bragg scattering from periodically microstructured light emitting diodes” by Lupton et al., Applied Physics Letters, Vol. 77, No. 21, Nov. 20, 2000, pp. 3340-3342. Brightness enhancement films having diffractive properties and surface and volume diffusers are described in WO0237568 A1 entitled “Brightness and Contrast Enhancement of Direct View Emissive Displays” by Chou et al., published May 10, 2002. The use of micro-cavity techniques is also known; for example, see “Sharply directed emission in organic electroluminescent diodes with an optical-microcavity structure” by Tsutsui et al., Applied Physics Letters 65, No. 15, Oct. 10, 1994, pp. 1868-1870. However, none of these approaches cause all, or nearly all, of the light produced to be emitted from the device. Moreover, such diffractive techniques cause a significant frequency dependence on the angle of emission so that the color of the light emitted from the device changes with the viewer's perspective.
Reflective structures surrounding a light-emitting area or pixel are referenced in U.S. Pat. No. 5,834,893 issued Nov. 10, 1998 to Bulovic et al. and describe the use of angled or slanted reflective walls at the edge of each pixel. Similarly, Forrest et al. describe pixels with slanted walls in U.S. Pat. No. 6,091,195 issued Jul. 18, 2000. These approaches use reflectors located at the edges of the light emitting areas. However, considerable light is still lost through absorption of the light as it travels laterally through the layers parallel to the substrate within a single pixel or light emitting area.
Scattering techniques are also known. Chou (International Publication Number WO 02/37580 A1) and Liu et al. (US 2001/0026124 A1) taught the use of a volume or surface scattering layer to improve light extraction. The scattering layer is applied next to the organic layers or on the outside surface of the glass substrate and has an optical index that matches these layers. Light emitted from the OLED device at higher than critical angle that would have otherwise been trapped can penetrate into the scattering layer and be scattered out of the device. The efficiency of the OLED device is thereby improved but still has deficiencies as explained below.
U.S. Pat. No. 6,787,796 entitled “Organic electroluminescent display device and method of manufacturing the same” by Do et al issued 20040907 describes an organic electroluminescent (EL) display device and a method of manufacturing the same. The organic EL device includes a substrate layer, a first electrode layer formed on the substrate layer, an organic layer formed on the first electrode layer, and a second electrode layer formed on the organic layer, wherein a light loss preventing layer having different refractive index areas is formed between layers of the organic EL device having a large difference in refractive index among the respective layers. US 2004/0217702 entitled “Light extracting designs for organic light emitting diodes” by Garner et al., similarly discloses use of microstructures to provide internal refractive index variations or internal or surface physical variations that function to perturb the propagation of internal waveguide modes within an OLED. When employed in a top-emitter embodiment, the use of an index-matched polymer adjacent the encapsulating cover is disclosed.
However, scattering techniques, by themselves, cause light to pass through the light-absorbing material layers multiple times where they are absorbed and converted to heat. Moreover, trapped light may propagate a considerable distance laterally through the cover, substrate, or organic layers before being scattered out of the device, thereby reducing the sharpness of the device in pillaged applications such as displays. For example, as illustrated in FIG. 14, a prior-art pillaged bottom-emitting OLED device may include a plurality of independently controlled pixels 30 and a scattering layer 22 located between the transparent first electrode 12 and the substrate 10. A light ray 5 emitted from the light-emitting layer 14 may be scattered multiple times while traveling through the substrate 10, organic layer(s) 14, and transparent first electrode 12 before it is emitted from the device. When the light ray 5 is finally emitted from the device, the light ray 5 may have traveled a considerable distance through the various device layers from the original pixel location where it originated to a remote pixel where it is emitted, thus reducing sharpness. Also, the amount of light emitted is reduced due to absorption of light in the various layers. Referring to FIG. 15, in a top-emitter configuration of a prior-art scattering OLED device light ray 5 travels through the cover in a manner analogous to the light ray 5 traveling through the substrate in FIG. 14. Note that because the organic layer(s) 14 are very thin relative to the substrate 10, cover, 20, and scattering layer 22, the light rays travel only a relatively insignificant distance through the organic layer(s) 14 and electrode 16.
Light-scattering layers used externally to an OLED device are described in U.S. Pat. No. 5,955,837 entitled “System with an active layer of a medium having light-scattering properties for flat-panel display devices” by Horikx, et al. This disclosure describes a scattering layer located on a substrate. Likewise, U.S. Pat. No. 6,777,871 entitled “Organic ElectroLuminescent Devices with Enhanced Light Extraction” by Duggal et al., describes the use of an output coupler comprising a composite layer having specific refractive indices and scattering properties. While useful for extracting light, this approach will only extract light that propagates in the substrate (illustrated with light ray 2) and will not extract light that propagates through the organic layers and electrodes (illustrated with light ray 3). Moreover, if applied to display devices, this structure will decrease the perceived sharpness of the display. Referring to FIG. 17, the sharpness of an active matrix OLED device employing a light-scattering layer coated on the substrate is illustrated. The average MTF (sharpness) of the device (in both horizontal and vertical directions) is plotted for an OLED device with the light-scattering layer and without the light scattering layer. As is shown, the device with the light-scattering layer is much less sharp than the device without the light scattering layer, although more light was extracted (not shown) from the OLED device with the light-scattering layer.
US 2004/0061136 entitled “Organic light emitting device having enhanced light extraction efficiency” by Tyan et al., describes an enhanced light extraction OLED device that includes a light scattering layer. In certain embodiments, a low index isolation layer (having an optical index substantially lower than that of the organic electroluminescent element) is employed adjacent to a reflective layer in combination with the light scattering layer to prevent low angle light from striking the reflective layer, and thereby minimize absorption losses due to multiple reflections from the reflective layer. The particular arrangements, however, may still result in reduced sharpness of the device.
There is a need therefore for an improved organic light-emitting diode device structure that avoids the problems noted above and improves the efficiency, sharpness, and brightness of the device.